In the manufacturer of gypsum board, whether it be wall board or ceiling board or used for some other purpose, it is desirable to use a low density gypsum to reduce the overall weight of the resulting board. The density of the gypsum can be reduced by introducing a foaming agent into the slurry that ultimately results in the core of the gypsum board. A problem with doing this, however, is that the low density gypsum does not adhere as well to the paper cover sheets that are typically used to produce the gypsum board.
One method of overcoming this adhesion problem apart from adding expensive adhesive or bond promoting agents to the gypsum slurry, is to coat the cover sheets with normal or higher density gypsum to form a bonding layer between the low density core and the paper cover sheets. Various methods have been tried to apply the higher density gypsum bonding layer to the cover sheets. One method is to spray the high density gypsum onto the cover sheets before applying the core gypsum. A difficulty with this approach, however, is that it is very difficult to get an even high density layer. The spraying apparatus is also prone to plugging problems.
Another approach is shown in the U.S. Camp Pat. No. 1,953,589. This patent shows the use of an oscillating and rotating roller that rubs the slurry into the cover sheet to make the slurry penetrate the cover sheet. The cover sheet must be backed up under the coating roller by a forming table or by a pressure roller in order for the coating roller to be able to apply sufficient pressure to rub the slurry into the cover sheet. A difficulty with this approach, however, is the high pressure required between the coating roller and the forming table or pressure roller. This creates paper break problems if foreign objects or lumps pass under the coating roller.
Another approach is to use multiple coating rollers to spread the high density gypsum over the cover sheet. An example of this is shown in the U.S. Brothers Pat. No. 2,940,505. In this patent, coating rollers bear against the cover sheets which are supported on flat table surfaces located beneath the coating rollers. A difficulty with this method, however, is that the high density gypsum tends to build up on the coating rollers. This can cause uneven coating thicknesses, or worse, lumps of partially set gypsum can form which get jammed beneath the coating rollers and cause paper breaks.
In U.S. Pat. No. 5,718,797 issued to John L. Phillips et al., the cover sheet passes beneath a counter-rotating coating roller, and a pressure roller located below the cover sheet and located upstream of the coating roller presses the cover sheet into engagement with the coating roller. The cover sheet so pressed against the coating roller causes the cover sheet to wipe the coating roller clean. While this may alleviate the problem of gypsum build up on the coating roller, there is still the problem of running the cover sheet through a high pressure nip between the coating and pressure rollers, which could cause paper breaks or other difficulties.
In the present invention, the high density layer is achieved by using a spreader roller without a backing or pressure roller. The spreader roller depresses the paper sheet below the forming table, and a combination of the roller speed and the tension in the paper sheet keeps the coating roller clean.